having since then been gradually cut out so as to sever these transverse valleys and divert their drainage in great part into its own channel.1

1 The erosion of the glacial period must not be forgotten. Loch Tay, like the other rock-inclosed Highland lakes, has had its basin scooped out, I believe, by land-ice; but the valley itself was probably there before the ice filled it. This subject will be discussed a few pages further on. With regard to the power of streams to shift their own course and cut their way to each other across a low watershed, it may be mentioned here in passing that the main watershed of the country between the Tweed and Clyde, crosses at one part a low valley through which it would be easy to cut a channel for the Clyde. Indeed, if good care were not taken of its banks, the Clyde would ere long dig the channel for itself and flow into the Tweed. In offering the above sketch of the origin of the Highland valleys, I make it chiefly as a suggestion in a subject full of difficulty, and standing much in need of thorough discussion. Though I am fully persuaded that these valleys are to be looked upon as the results not of subterranean movements, but of subaërial denudation, I have still very much to learn as to the way in which the process of excavation was carried on.

CHAPTER VII.

INFLUENCE OF ANCIENT GLACIERS AND ICEBERGS ON HIGHLAND SCENERY.

FROM the foregoing narrative it appears that a wide sea-worn tract of land has, since the time of the Lower Old Red Sandstone, been channelled out by running water, and general atmospheric waste, into systems of valleys, between which the intervening ground, having suffered less denudation, rises up into ranges of hills and mountains-the whole forming the framework of the Highlands of Scotland. We have now to trace the impress of another agencythat of glacier-ice and icebergs, no longer at work in our country, but which once distinguished the British Isles as markedly as the Arctic ice-fields characterise Greenland, and which played an important part in the changes that brought about the present scenery of the country. It is now well ascertained that during a comparatively recent geological period, the climate of the northern hemisphere was much colder than at present, and that in the British Islands, as well as in other countries where glaciers are now unknown, the land was enveloped in snow and ice. This part of the geological

record is known as the Glacial Period. There is a growing belief among geologists that it was not an abnormal condition of things, but that in the past history of the globe there have been older cold periods succeeding each other after wide intervals.

In following the track of the ancient Scottish glaciers and icebergs, and noting how much they have influenced the scenery of the country, it must be borne in mind that the present great leading features of mountain and valley had been fixed before the ice-age began. The Highland hills rose then above the glens and valleys as they still do, and the glens and valleys wound in the same tortuous course towards the sea. The minor outlines of the surface, however, were, perhaps, in many respects, unlike those which it wears at the present day. There may have been, and probably there was, far more angularity and ruggedness about the contour of that ancient land. The passing of the long glacial period did much to remove such irregularities and smooth the general surface of the country. But though the ice must have worn down the valleys, it did not make them. They are to be attributed, I believe, to that earlier process of sub-aërial denudation which has just been sketched. Keeping in recollection, therevalley were grouped into their present arrangement before the ice began to settle down upon them, let us look for a little at the evi

fore, that hill and

dence from which this strange chapter in the country's history is deciphered.1

Track of the First Great Ice-sheet of the Glacial Period.

The surface of Scotland, like that of Ireland and large tracts of England, as well as the whole of Scandinavia and northern Europe, has a peculiar contour, visible almost everywhere, irrespective of the nature of the rock on which it shows itself, and therefore to be regarded as the result of one great process, acting upon all the rocks alike, long after they had been formed. This contour consists in a rounding and smoothing of the hills and valleys into long flowing outlines. What were once prominent crags have been ground down into undulating or pillow-shaped knolls, and deep hollows or gentler depressions have been worn in the solid rock, not at random, but in a recognisable system. These features may be seen throughout the whole of the country. At present we are concerned only with their development in the Highlands. It may seem paradoxical to speak of the well-known rugged Highland mountains as showing traces of a general smoothing of their surface. But such is really the

case.

There may be places, indeed, where from

1 See Memoir on the Glacial Drift of Scotland; and North British Review, Vol. xxxix, p. 286, et seq.

height, or steepness, or some other cause, the smoothed surface was never communicated; and there is everywhere a constantly progressing destruction of that peculiar outline; the rains, springs, and frosts are re-asserting their sway, and carving anew upon the country its ancient ruggedness. Nevertheless, to an eye which has learnt to distinguish the characteristic flowing lines, there are not many landscapes in the kingdom where they cannot be traced. Even in the wildest Highland scenery, where the casual tourist may see nothing but thunder-riven crags and precipices, and glens blocked up with their ruins

"Precipitous black, jagged rocks,

For ever shattered and the same for ever,"

the geologist can often detect traces of the same universal smoothing and moulding. Nay, it is precisely amid such scenes that he is most vividly impressed with the fact that the surface of the country has been ground down by a vast general agent, for he there sees what are the natural outlines which the rocks assume when left to the ordinary attacks of the elements. Instead of smooth undulating outlines he notes craggy precipices and scars, here and there red and fresh, where the last winter's frosts have let loose masses of rock into the valleys below. He can trace how in this way, the hand of Nature is once more